SIMD functions to apply toupper/tolower to each character in a string

change_case_simd.c

/* Created by easyaspi314. Released into the public domain. */

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifdef __SSE2__
#include <immintrin.h>

/* A SIMD function for SSE2 which changes all uppercase ASCII digits
 * to lowercase. */
void StringToLower(char *str)
{
    size_t len = strlen(str);
    const __m128i asciiA = _mm_set1_epi8('A');
    const __m128i asciiZ = _mm_set1_epi8('Z' + 1);
    const __m128i diff = _mm_set1_epi8('a' - 'A');
    while (len >= 16) {
        __m128i inp = _mm_loadu_si128((__m128i*)str);
        /* > 'A': 0xff, < 'A': 0x00 */
        __m128i greaterThanA = _mm_cmpgt_epi8(inp, asciiA);
        /* <= 'Z': 0xff, > 'Z': 0x00 */
        __m128i lessEqualZ = _mm_cmplt_epi8(inp, asciiZ);
        /* 'Z' >= x >= 'A': 0xFF, else 0x00 */
        __m128i mask = _mm_and_si128(greaterThanA, lessEqualZ);
        /* 'Z' >= x >= 'A': 'a' - 'A', else 0x00 */
        __m128i toAdd = _mm_and_si128(mask, diff);
        /* add to change to lowercase */
        __m128i added = _mm_add_epi8(inp, toAdd);
        _mm_storeu_si128((__m128i *)str, added);
        len -= 16;
        str += 16;
    }
    while (len-- > 0) {
        *str = tolower(*str);
        ++str;
    }
}

/* Same, but to uppercase. */
void StringToUpper(char *str)
{
    size_t len = strlen(str);
    const __m128i asciia = _mm_set1_epi8('a');
    const __m128i asciiz = _mm_set1_epi8('z' + 1);
    const __m128i diff = _mm_set1_epi8('a' - 'A');
    while (len >= 16) {
        __m128i inp = _mm_loadu_si128((__m128i*)str);
        /* > 'a': 0xff, < 'a': 0x00 */
        __m128i greaterThana = _mm_cmpgt_epi8(inp, asciia);
        /* <= 'z': 0xff, > 'z': 0x00 */
        __m128i lessEqualz = _mm_cmplt_epi8(inp, asciiz);
        /* 'z' >= x >= 'a': 0xFF, else 0x00 */
        __m128i mask = _mm_and_si128(greaterThana, lessEqualz);
        /* 'z' >= x >= 'a': 'a' - 'A', else 0x00 */
        __m128i toSub = _mm_and_si128(mask, diff);
        /* subtract to change to uppercase */
        __m128i added = _mm_sub_epi8(inp, toSub);
        _mm_storeu_si128((__m128i *)str, added);
        len -= 16;
        str += 16;
    }
    while (len-- > 0) {
        *str = toupper(*str);
        ++str;
    }
}
#elif defined(__ARM_NEON) || defined(__ARM_NEON__)
#include <arm_neon.h>

/* Literally the exact same code as above, but for NEON. */
void StringToLower(char *str)
{
    size_t len = strlen(str);
    const uint8x16_t asciiA = vdupq_n_u8('A');
    const uint8x16_t asciiZ = vdupq_n_u8('Z' + 1);
    const uint8x16_t diff = vdupq_n_u8('a' - 'A');
    while (len >= 16) {
        uint8x16_t inp = vld1q_u8((uint8_t *)str);
        uint8x16_t greaterThanA = vcgtq_u8(inp, asciiA);
        uint8x16_t lessEqualZ = vcltq_u8(inp, asciiZ);
        uint8x16_t mask = vandq_u8(greaterThanA, lessEqualZ);
        uint8x16_t toAdd = vandq_u8(mask, diff);
        uint8x16_t added = vaddq_u8(inp, toAdd);
        vst1q_u8((uint8_t *)str, added);
        len -= 16;
        str += 16;
    }
    while (len-- > 0) {
        *str = tolower(*str);
        ++str;
    }
}

/* Literally the exact same code as above, but for NEON. */
void StringToUpper(char *str)
{
    size_t len = strlen(str);
    const uint8x16_t asciia = vdupq_n_u8('a');
    const uint8x16_t asciiz = vdupq_n_u8('z' + 1);
    const uint8x16_t diff = vdupq_n_u8('a' - 'A');
    while (len >= 16) {
        uint8x16_t inp = vld1q_u8((uint8_t *)str);
        uint8x16_t greaterThana = vcgtq_u8(inp, asciia);
        uint8x16_t lessEqualz = vcltq_u8(inp, asciiz);
        uint8x16_t mask = vandq_u8(greaterThana, lessEqualz);
        uint8x16_t toSub = vandq_u8(mask, diff);
        uint8x16_t added = vsubq_u8(inp, toSub);
        vst1q_u8((uint8_t *)str, added);
        len -= 16;
        str += 16;
    }
    while (len-- > 0) {
        *str = toupper(*str);
        ++str;
    }
}
#else
/* Just go scalar. */
void StringToLower(char *str)
{
    size_t len = strlen(str);

    while (len-- > 0) {
        *str = tolower(*str);
        ++str;
    }
}

void StringToUpper(char *str)
{
    size_t len = strlen(str);

    while (len-- > 0) {
        *str = toupper(*str);
        ++str;
    }
}
#endif

#ifdef TEST
#include <assert.h>
int main()
{
    char str[] = "Hello world 12345 HI AZaz";
    StringToLower(str);
    assert(!strcmp(str, "hello world 12345 hi azaz"));
    puts(str);

    StringToUpper(str);
    assert(!strcmp(str, "HELLO WORLD 12345 HI AZAZ"));
    puts(str);
}
#endif

Hi, thanks for the useful code example.

It seems TEST does not work as intended. Since strlen(str) == 26, the last part ( HI AZaz) is processed with scalar loop.
If it passes str[] = "Hello world 12345 hi ABXYZ abxyz" or longer string, TEST will fail properly.

As for actual functions, as @bb1950328 suggested, the following patch may work:

patch

void StringToLower(char *str)
{
    size_t len = strlen(str);
-   const __m128i asciiA = _mm_set1_epi8('A');
+   const __m128i asciiA = _mm_set1_epi8('A' - 1);
    const __m128i asciiZ = _mm_set1_epi8('Z' + 1);
...

void StringToUpper(char *str)
{
    size_t len = strlen(str);
-   const __m128i asciia = _mm_set1_epi8('a');
+   const __m128i asciia = _mm_set1_epi8('a' - 1);
    const __m128i asciiz = _mm_set1_epi8('z' + 1);
...

void StringToLower(char *str)
{
    size_t len = strlen(str);
-   const uint8x16_t asciiA = vdupq_n_u8('A');
+   const uint8x16_t asciiA = vdupq_n_u8('A' - 1);
    const uint8x16_t asciiZ = vdupq_n_u8('Z' + 1);
...

void StringToUpper(char *str)
{
    size_t len = strlen(str);
-   const uint8x16_t asciia = vdupq_n_u8('a');
+   const uint8x16_t asciia = vdupq_n_u8('a' - 1);
    const uint8x16_t asciiz = vdupq_n_u8('z' + 1);
...

int main()
{
-   char str[] = "Hello world 12345 HI AZaz";
+   char str[] = "Hello world 12345 HI ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz #PAD#";
    StringToLower(str);
-   assert(!strcmp(str, "hello world 12345 hi azaz"));
+   assert(!strcmp(str, "hello world 12345 hi abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz #pad#"));
    puts(str);

    StringToUpper(str);
-   assert(!strcmp(str, "HELLO WORLD 12345 HI AZAZ"));
+   assert(!strcmp(str, "HELLO WORLD 12345 HI ABCDEFGHIJKLMNOPQRSTUVWXYZ ABCDEFGHIJKLMNOPQRSTUVWXYZ #PAD#"));
    puts(str);
...